The “chirp” is bright
and bird-like, its pitch rising at the end as though it’s asking a question. To
an untrained ear, it resembles a sound effect from a video game more than the
faint, billion-year-old echo of the collision of two black holes. But to the
trained ear of experimental physicist, it is the opening note of a cosmic
symphony. On Thursday, for the first time in history, scientists announced that
they are able to hear the ripples in the space-time continuum that are produced
by cosmic events — called gravitational waves. The discovery opens up a new
field of scientific research, one in which physicists listen for the secrets of
the universe rather than looking for them. “Until this moment, we had our eyes
on the sky and we couldn’t hear the music,” said Columbia University
astrophysicist Szabolcs Márka, a member of the discovery team, according to the
Associated
Press. “The skies will never be the same.”
Thursday’s moment of
revelation has its roots almost a century earlier, in 1916, when Albert Einstein
predicted the existence of gravitational waves as part of his ground-breaking
theory of general relativity. The intervening years included brush-offs and
boondoggles, false hope, reversals of opinion, an unlikely decision to take a
$272 million risk, and a flash of serendipity that seemed too miraculous to be
real — but wasn’t. Here’s how it all happened. In 1915, Einstein gave a series
of lectures on his theory of general relativity, asserting that space and time
form a continuum that gets distorted by anything with mass. The effect of that
warping is gravity — the force that compels everything, from light to planets
to apples dropping from a tree, to follow a curved path through space.
Gravitational waves, which he proposed the following year, are something of a
corollary to that theory. If spacetime is the fabric of the cosmos, then huge
events in the cosmos — like a pair of black holes banging into each other —
must send ripples through it, the way a the fabric of trampoline would vibrate
if you bounced two bowling balls onto it. Those ripples are gravitational
waves, and they’re all around us, causing time and space to minutely squeeze
and expand without us ever noticing. They’re so weak as to be almost
undetectable, and yet, according to Einstein’s math at least, they must be
there.
But like the entire
theory of general relativity, gravitational waves were just a thought
experiment, just equations on paper, still unproven by real-world events. And
both were controversial. Some people believe that the initial skepticism about
Einstein’s theory, plus blatant anti-Semitism — some prominent German
physicists called it “world-bluffing Jewish physics,” according to Discover
Magazine — explains why he never got the Nobel Prize for it (he was eventually
awarded the the 1921 Nobel Prize in Physics for his explanation of the
photoelectric effect). So scientists came up with a series of tests of general
relativity. The biggest took place in 1919, when British physicist Sir Albert
Eddington took advantage of a solar eclipse to see if light from stars bent as
it made its way around the Sun (as Einstein said it should). It did, surprising
Einstein not in the slightest. According to Cosmos, when he was asked what he
would have done if the measurements had discredited his theory, the famous
physicist replied: “In that case, I would have to feel sorry for God, because
the theory is correct.”
One by one, successive
experiments proved other aspects of general relativity to be true, until all
but one were validated. No one, not even Einstein, could find evidence of
gravitational waves. Eddington, who so enthusiastically demonstrated Einstein’s
theory of relativity, declared that gravitational waves were a mathematical
phantom, rather than a physical phenomenon. The only attribute the waves seemed
to have, he snidely remarked, was the ability to travel “at the speed of
thought.” In the end, Einstein himself had doubts. Twice he reversed himself
and declared that gravitational waves were non-existent, before turning another
about-face and concluding that they really were real. Time passed. A global
depression happened, followed by a global war. A reeling and then resurgent
world turned its scientific eye toward other prizes: bombs, rockets, a polio
vaccine. Then, in the 1960s, an engineering professor at the University of
Maryland decided he would try his hand at capturing the waves that had so
eluded the man who first conceived them. The engineer, Joe Weber, set up two
aluminum cylinders in vacuums in labs in Maryland and Chicago. The tiny ripples
of gravitational waves would cause the bars to ring like a bell, he reasoned,
and if both bars rang at once, then he must have found something.
Weber declared his
first discovery in 1969, according to the New Yorker. The news was met with
celebration, then skepticism, as other laboratories around the country failed
to replicate his experiment. Weber never gave up on his project, continuing to
claim new detections until he died in 2000. But others did. It didn’t help that
gravitational waves supposedly detected by a South Pole telescope in 2014
turned out to be merely a product of cosmic dust. People were inclined to
believe, physicist Rainer Weiss told the New Yorker, that gravitational-wave
hunters were “all liars and not careful, and God knows what.” Weiss would prove
them wrong. Now 83, he was a professor at the Massachusetts Institute of
Technology when Weber first started publishing his purported discoveries. “I
couldn’t for the life of me understand the thing he was doing,” he said in a
Q&A for the university website. “That was my quandary at the time, and
that’s when the invention was made.”
Weiss tried to think of
the simplest way to explain to his students how gravitational waves might be
detected, and came up with this: Build an immense, L-shaped tunnel with each
leg an equal length and a mirror at the far ends, then install two lasers in
the crook of the L. The beams of light should travel down the tunnels, bounce
off the mirrors, and return to their origin at the same time. But if a
gravitational wave was passing through, spacetime would be slightly distorted,
and one light beam would arrive before the other. If you then measure that
discrepancy, you can figure out the shape of the wave, then play it back as
audio. Suddenly, you’re listening to a recording of the universe. That idea
would eventually become the Laser Interferometer Gravitational-Wave Observatory
(LIGO), the pair of colossal facilities in Washington and Louisiana where the
discovery announced Thursday was made. For one thing, even though gravitational
waves are all around us, only the most profound events in the universe produce
ripples dramatic enough to be measurable on Earth — and even those are very,
very faint. For another, an instrument of the size and strength that Weiss
desired would require a host of innovations that hadn’t even been created yet:
state-of-the-art mirrors, advanced lasers, supremely powerful vacuums, a way to
isolate the instruments from even the faintest outside interference that was
better than anything that had existed before. The L tunnel would also have to
be long — we’re talking miles here — in order for the misalignment of the light
beams to be detectable. Building this instrument was not going to be easy, and
it was not going to be cheap.
And there would need to
be two of them. The principles of good scientific inquiry, which requires that
results be duplicated, demanded it. It took a few decades and a number of
proposals, but in 1990 the National Science Foundation finally bit. Weiss and
his colleagues could have $272 million for their research. “It should never
have been built,” Rich Isaacson, a program officer at the National Science
Foundation at the time, told the New Yorker. “There was every reason to imagine
[LIGO] was going to fail,” he also said. But it didn’t. Twenty-one years and
several upgrades after ground was broken on the first LIGO lab, the instruments
finally found something on Sept. 14, 2015. Like most scientific discoveries,
this one started not with a “Eureka,” but a “Huh, that’s weird.”
That’s what Marco
Drago, a soft-spoken post-doc sitting at a desk in Hanover, Germany, thought
when he saw an email pop up in his inbox. It was from a computer program that
sorts through data from LIGO to detect evidence of gravitational waves. Drago
gets those messages almost daily, he told Science Magazine — anytime the
program picks up an interesting-seeming signal. This was a big one. Almost too
big, considering that Sept. 14 was the very first day of official observations
for the newly-revamped LIGO instruments. Drago could only assume that the
pronounced blip in his data was a “blind injection,” an artificial signal
introduced to the system to keep researchers on their toes, make sure that
they’re able to treat a exciting-seeming with the appropriate amount of
scrutiny. But the injection system wasn’t supposed to be running yet, since
research had just started. After about an hour of seeking some other
explanation, Drago sent an email to the whole LIGO collaboration, he told Science:
Was there an injection today? No, said an email sent that afternoon. Something
else must have caused it. But no one had an explanation for the signal. Unless,
of course, it was what they were looking for all along.
Chad Hanna, an
assistant professor of physics at Pennsylvania State University who was also
part of the LIGO team, blanched as he read the successive emails about the
weird signal. He and his colleagues had joked about their instruments detecting
something on Day One, he wrote for the Conversation, but no one imagined that
could really happen. “My reaction was, ‘Wow!’” LIGO executive director David
Reitze said Thursday, as he recalled seeing the data for the first time. “I
couldn’t believe it.” Yet, as the weeks wore on and after an exhaustive battery
of tests — including an investigation to make sure that the signal wasn’t the
product of some ill-conceived prank or hoax — all the other possible sources of
the signal were rejected. Only one remained: Long ago and far from Earth, a
pair of black holes began spiraling around one another, getting closer and
closer, moving faster and faster, whirling the spacetime around them, until,
suddenly, they collided. A billion years later, a ripple from that dramatic
collision passed through the two LIGO facilities, first in Louisiana, then,
after seconds, in Washington.
The realization of what
they’d found hit the LIGO collaborators differently. For some, it was a
vindication — for themselves as well as the men who inspired them: “Einstein
would be beaming,” Kip Thorne, a Cal-Tech astrophysicist and co-founder of the
project with Weiss, said at the press conference Thursday. After the briefing
he also credited Weber, the UMD professor: “It does validate Weber in a way
that’s significant. He was the only person in that era who thought that this
could be possible.” Thorne told Scientific American he’s feeling a sense of
“profound satisfaction” about the discovery. “I knew today would come and it
finally did,” he said. For Weiss, who had invested half his life in the search
for gravitational waves, there’s just an overpowering sense of relief. “There’s
a monkey that’s been sitting on my shoulder for 40 years, and he’s been
nattering in my ear and saying, ‘Ehhh, how do you know this is really going to
work? You’ve gotten a whole bunch of people involved. Suppose it never works
right?'” he told MIT. “And suddenly, he’s jumped off.”
But the mood Thursday
was mostly one of awe, and joy, and excitement to see what comes next. Neil
deGrasse Tyson, the director of the Hayden Planetarium at the American Museum
of Natural History and celebrity astrophysicist, joined a gathering of Columbia
University scientists who had been involved in the LIGO project. They cheered
as they watched the Washington, D.C., press conference where Reitze announced
the find. “One hundred years feels like a lifetime but over the course of
scientific exploration it’s not that long,” Tyson told Scientific American
about the long search for gravitational waves. “I lay awake at night wondering
what brilliant thoughts people have today that will take 100 years to reveal
themselves.”
